Appropriate sample sizes for human and animal studies are necessary to support the validity of research conclusions and assure the efficient use of resources. Investigators often have no idea how to select a sample size, and even statisticians face a large, scattered literature, incomplete tables, and software that is neither user-friendly nor comprehensive. Our research goal is to develop a PC Windows based software product and related textbook to facilitate planning study sample size and power by research investigators and statisticians. The software will be modular and interactive; a structured inquiry and response format will assist investigators in specification of study design, outcome variable, and analysis goals. It will have tabular entry screens and tabular and graphic displays of power results. Help screens will provide documentation and access to interactive dynamic tutorials to aid users in specifying designs and effect sizes. In Phase II, software design specifications will be completed, specifications for module components will be detailed, and code for these modules will be developed. Statistical research will develop methods or problems receiving of equivalence and multiple goals, Help and tutorial systems and the textbook will be drafted, and feedback from potential users integrated.